The process of joining of two discrete pieces of material, especially metals, is undoubtedly fundamental to engineering and is often the basis for the construction of various engineered structures. Indeed, the ability to join discrete pieces of metal has enabled the production of a host of structures that would not otherwise be capable of manufacture.
A number of methods have been developed to join two materials, each being useful in certain applications. For example, welding is one such method. Generally, welding involves: placing two metallic pieces in contact with one another; locally melting them at their point of contact; allowing the molten metals to coalesce; and then allowing the newly joined configuration to cool and solidify. Ideally, since the metals solidify after having coalesced into one another, the strength of the bond will be at least as great as, if not greater than, that of either constituent metal. A filler material may also be utilized during the melting stage to facilitate the welding process.
Many techniques can be employed to weld two materials together, including spot-welding, soldering and brazing. Spot welding is a common technique that is effectuated by passing a current through the materials to be joined. The resistance inherent to the joint causes the temperature of the joint to be raised to the extent that the constituent metals melt and coalesce into one another.
Soldering is another prominent method used to join two materials. Unlike welding, soldering does not require melting either of the constituent metallic pieces. Instead, the pieces are arranged in a desired to-be-joined configuration, and an additional filler metallic alloy (“solder”) is placed at the desired point of joining between the constituent metals and melted. The molten solder locally envelopes both metallic pieces, solidifies as it cools, and thereby affixes the two constituent metals together. Note that the solder has to have a melting point lower than either of the constituent metals so that the process of melting the solder does not simultaneously melt any constituent metal. Finally, brazing is yet another prominent method used to join two metals. Brazing is like soldering, except that the filler metallic alloy (“braze alloy”) typically has a much higher melting point than typical solder. Thus, brazing and soldering are used in different application.
Whereas methods of joining have long been in use, Bulk Metallic Glasses (“BMGs”) are a relatively new engineering development. BMGs are metallic alloys that do not have a crystalline structure; instead, like glass, their structure is amorphous. (See e.g., U.S. Pat. Pub. US 2009/0236017 A1, the disclosure of which is incorporated herein by reference.) They are generally formed by raising specific alloys above their respective glass transition temperature, and cooling them at a sufficiently fast cooling rate (“critical cooling rate”) so that they do not have time to crystallize while above the glass transition temperature; instead, they are cooled quickly enough that they solidify in an amorphous glass-like structure (i.e. re-vitrify). BMGs have a number of beneficial material properties that make them viable for use in any number of engineering applications—some of these properties include: high strength, elasticity, corrosion resistance, and processability from the molten state.
Although BMGs have improved materials properties, their non-crystalline microstructure makes them susceptible to damage by conventional joining techniques. (See e.g., U.S. Pat. No. 4,115,682, the disclosure of which is incorporated herein by reference.) Specifically, conventional joining techniques typically require significantly heating the joint to the extent that some material (e.g., the constituent materials, solder, or braze alloy) is melted. Since conventional joining techniques are typically unconcerned with achieving a critical cooling rate, they risk annealing the amorphous microstructure of BMGs, and thereby eliminating their useful properties. (See e.g., U.S. Pat. No. 4,115,682, cited above.) Fortunately, specific methods for joining these BMGs have been developed. For instance, U.S. Pat. No. 4,115,682 discloses using a process akin to spot welding that specifically ensures that after heating is applied to the joint, it is allowed to cool quickly enough to enable the formation of the amorphous structure. Yavari et al. also disclose a method similar to spot welding to join to BMGs together. (See Yavari et al. Materials Research Society Symposium and Proceedings, 644 (2001) L12-20-1, the disclosure of which is incorporated herein by reference.) Similarly, U.S. Pat. No. 6,818,078 discloses a method of joining a BMG with a conventional crystalline metal with a different melting point by “melting” the material with the lower melting temperature and casting it against the other material—in the case that the material to be melted is the BMG, the BMG must then be cooled sufficiently fast to re-form the amorphous structure. (See U.S. Pat. No. 6,818,078, the disclosure of which is incorporated herein by reference.)
Even though BMGs possess promising material traits, they also have notable shortcomings that limit their viability as engineering materials. For instance, typical BMGs lack appreciable tensile ductility—instead, typical BMGs fail ungracefully exhibiting essentially zero plastic tensile strain. (See e.g., U.S. Pat. No. 7,883,592, the disclosure of which is incorporated herein by reference.) Thus, BMG Composites have been developed to address these deficiencies. BMG Composites are materials that have BMGs as an underlying matrix, but also include at least one other phase, the presence of which is understood to enhance material properties. Moreover, U.S. Pat. Pub. 2011/0203704 discloses how to precisely develop secondary phases within a BMG matrix in a highly controlled manner so as to optimize the composite's material properties. Specifically, it discusses methods for developing inhomogeneities within a BMG matrix that are of specific stiffness and size such that they promote shear band intersection and multiplication thereby inhibiting crack formation and propagation. As such, they allow for vastly improved properties. (See U.S. Pat. Pub. 2011/0203704, the disclosure of which is incorporated herein by reference.)
As BMG Composites are essentially an enhanced version of BMGs, they are even more promising for a host of engineering applications. However, methods for adjoining BMG composites that preserve their vital secondary phase microstructure have yet to be developed. Therefore, there arises a need to develop methods of joining BMG Composites that specifically preserve the secondary phase microstructures that underlie their enhanced material properties.